Web page error

[pcbinternet]

I use to be on a reseller account so i have no idea what iis version the company was using, but all worked ok. I have now gone on to a vps account which runs iis 6. Now a curious problem has arisen. In one of my sites its creates invoices and then emails the respective clients, in that email is an html link something like http://mysite.co.uk/makepayment.asp?o=1234&b=y. This link is then clicked and then the client can make an online payment. This worked fine on old machine however when the link is clicked now instead of showing the above page it >shows the index file stored in that directory.(If i dont close the broswer window and click the link a scond time it works fine. ) I have disabled default document for the web site in an attempt to isolate the problem, this has no effect though. Can anybody point me in the right direction with this problem .